Industrial Applications 108 Hydration structure around CO 2 captured in aqueous amine solutions observed by high energy X-ray scattering Carbon dioxide is widely recognized as a major greenhouse gas that causes the global warming problem. One approach to reducing CO 2 emission is CO 2 capture at thermal plants. Among various technologies for CO 2 capture, the chemical absorption method [1] using aqueous amine solutions is the closest to commercialization for large-scale plants. The chemical absorption method is based on a reversible chemical reaction between CO 2 and an aqueous amine solution. CO 2 in flue gas is separated into the solution by contact with the CO 2 -lean solution. After absorption, CO 2 is released from the CO 2 -rich solution by heating. Then, the regenerated solution is used again in the absorption process. Although some chemical absorption plants for factory-scale CO 2 emission sources have been under commercial operation, plants for large-scale CO 2 emission sources such as thermal plants are still under development. One significant way to improve chemical absorption is to develop a more efficient solution to be realized by such characteristics as high absorption capacity, a high absorption rate and a small thermal energy for regeneration. These properties depend on the hydration structure of chemical species bound with CO 2 . Better understanding of the structure can bring a new insight into the improvement of the solution performance. However, the analysis of the hydration structure has some difficulties because the materials for the analysis are liquid systems. We have been applying the high energy X-ray scattering method using SPring-8 to the analysis of the hydration structure and conformation of amine molecules [2-4]. Compared with spectroscopic methods, the X-ray scattering method has an advantage of the capacity to derive molecular structure directly through a distribution function. In this report, we describe the results obtained using 30 wt% monoethanolamine (MEA) aqueous solution before and after CO 2 absorption. MEA is a primary amine expressed in the chemical formula of NH 2 CH 2 CH 2 OH, and is a typical and fundamental amine used for the chemical absorption method. Measurements were carried out at the undulator beamline BL16XU . The incident X-ray wavelength was 0.3356 Å (36.94 keV). The scattered X-ray intensity from the sample solution was measured over a 2 θ range of 0.5 ≤ 2 θ ≤ 80°. The molar fractions of the sample solutions before and after CO 2 absorption were (MEA) 0.112 (H 2 O) 0.888 (CO 2 ) x ( x = 0 and 0.058, respectively). The sample solution was sealed in a flat plate acrylate resin cell with a thickness of 2 mm, which had X-ray transmission windows, each made of a Kapton film with a thickness of 25 μ m. An empty cell was also measured for background intensity correction. The total exposure time was about 4–5 h for each sample solution. F i g u r e 1 s h o w s t h e o b s e r v e d d i s t r i b u t i o n function g ( r ) of the sample solutions. In Fig. 1, the distribution function of a water sample is also shown for comparison. Three dominant peaks at r = 1.0, 1.5, and 2.8 Å are observed in g ( r ) for the amine solution samples. The peak at r = 1.0 Å can mainly be ascribed to the intramolecular interaction of O– H within the water molecule. The peak at r = 1.5 Å is attributed to intramolecular interactions (mainly C – C, C – N, and C – O) in the amine (carbamate) molecule. The peak at r = 2.8 Å can be attributed mainly to the intermolecular hydrogen – bonded O … O interaction between the nearest neighbor water molecules. This peak also involves hydrogen–bonded O…O and N…O interactions concerning the nearest neighbor intermolecular amine…amine and amine… water interactions. Thus, atom pairs of the molecules existing in the sample solutions are clearly observed. H o w e v e r, v a r i a t i o n s b e f o r e a n d a f t e r C O 2 absorption are not clear. In order to clarify structural information on the captured CO 2 , we derived a difference distribution function between before and after CO 2 absorption, Δ g CO 2 ( r ) , as shown in Fig. 2. Peaks at r = 1.2 Å and 2.2 Å are obviously observed in the difference distribution function. These peaks Fig. 1. Observed distribution functions. r (Å) water MEA solution before CO 2 absorption (+2) MEA solution after CO 2 absorption (+4) g ( r ) 0 –1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 1 2 3 4 5 6 7 8 9 10 109 are mainly attributed to the intramolecular C – O and nonbonding O…O interactions of captured CO 2 , respectively. The difference distribution function in Fig. 2 involves both intramolecular interactions within m o l e c u l e s t h a t b i n d t h e c a p t u r e d C O 2 a n d intermolecular interactions between the captured CO 2 and its neighboring molecules. By subtracting the intramolecular contributions from the total distribution function, the intermolecular distribution function can be obtained. An NMR analysis showed that absorbed CO 2 molecules in the sample exist as MEA carbamate of 86 mol%, HCO 3 − /CO 3 2− of 13 mol%, and MEA carbonate of 1 mol%. The molecular s t r u c t u r e o f M E A c a r b a m a t e w a s c a l c u l a t e d theoretically using DFT calculation. Five stable carbamate structures were obtained. Although the molar ratio between HCO 3 − and CO 3 2− is unknown, we considered the HCO 3 − molecule only and used the structure in a single crystal of NaHCO 3 . The amine carbonate molecule was ignored because of its low molar ratio. From these molecular structures, we calculated the intramolecular interactions. By subtracting the intramolecular interactions from the total interactions, we obtained the intermolecular distribution function, Δ g inter CO 2 ( r ) , as shown in Fig. 3. The distribution functions in Fig. 3(a – e) were derived f r o m t h e M E A c a r b a m a t e s t r u c t u r e s ( a ) – ( e ) , respectively. On the basis of any MEA carbamate structure, a broad peak at around r = 0.35 nm can be observed. By the detailed analysis, it was revealed that this peak originated from water molecules that formed hydrogen bonds with the captured CO 2 . This result suggests that new hydrogen bonds are formed between the captured CO 2 molecules and the neighboring water molecules. Further study on other amine solutions showed that CO 2 captured as either amine carbamate or HCO 3 − /CO 3 2− formed hydrogen bonds with water molecules. Understanding of the hydrated structure at the molecular level will contribute to the development of new absorbing solutions with higher performance. Hiroshi Deguchi a, *, Noriko Yamazaki b and Yasuo Kameda c a Power Engineering R&D Center, Kansai Electric Power Co., Inc. b Advanced Technology Research Center, Mitsubishi Heavy Industries, Ltd. c Dept. of Material and Biological Chemistry, Yamagata University *E-mail: deguchi.hiroshi@c4.kepco.co.jp References [1] J.D. Figueroa et al .: Int. J. Greenhouse Gas Control 2 (2008) 9. [2] H. Deguchi et al .: Ind. Eng. Chem. Res. 49 (2010) 6. [3] H. Deguchi, Y. Kubota, H. Furukawa, Y. Yagi, Y. Imai, M. Tatsumi, N. Yamazaki, N. Watari, T. Hirata, N. Matubayasi, Y. Kameda: Int. J. Greenhouse Gas Control 5 (2011) 1533. [4] Y. Kameda et al .: Bull. Chem. Soc. Jpn. 86 (2013) 99. Fig. 2. Difference distribution function observed between before and after CO 2 absorption. Fig. 3. Intermolecular distribution functions around captured CO 2 . The distribution functions a–e were derived from the MEA carbamate structures (a)–(e) , respectively. r (Å) 0 1 2 3 4 5 6 Δ g CO2 ( r ) –2 –1 1 2 3 4 0 r (Å) a (+8) b (+6) c (+4) d (+2) e (e) (d) (c) (b) (a) MEA Carbamate Structure 0 1 2 3 4 5 6 N C O H –1 1 2 3 4 5 6 7 9 8 10 11 0 Δ g inter ( r ) CO 2
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